Interpretive Summary: Increasing carbon dioxide in the air has been shown to change the chemistry in plant leaves. Leaves harbor fungi that produce spores that can cause serious allergies in humans. In our research, we found that exposing the forage grass timothy to increasing levels of carbon dioxide resulted in an increased number of spores produced by fungi growing on the grass. The fungi used in these experiments included a common allergy-inducing species and disease-causing species. For the allergy-producing species the number of spores increased three times at high levels of carbon dioxide. The increase in fungal spore production may contribute to the increasing prevalence of asthma. This research will be of interest to plant scientists, health care providers and the general public.

Technical Abstract:
Background: Although the association between rising levels of carbon dioxide, the principle anthropogenic greenhouse gas, and pollen production has been established, few data are available regarding the function of rising carbon dioxide on quantitative or qualitative changes in allergenic fungal sporulation on plant material.
Objective: The purpose of the study was to examine the effects of elevated atmospheric carbon dioxide on the quantity and quality of spores produced by Alternaria alternata, an allergenic fungus, growing on timothy hay.
Methods: Timothy grass (Phleum pratense) was grown at four levels of carbon dioxide. Leaves were used as substrate for the growth of A. alternata. The abundance and size of the spores produced were quantified via microscopy, and allergenic protein content was quantified using ELISA.
Results: Timothy grass leaf dry weight and carbon-to-nitrogen ratio both increased at higher levels of carbon dioxide. Leaf carbon-to-nitrogen ratio was positively correlated with the log of A. alternata spores produced per gram of leaf, and negatively correlated with spore content of allergenic protein. On a per-plant basis, A. alternata spore production was increased nearly three-fold at the higher levels of carbon dioxide.
Conclusions: Rising levels of carbon dioxide often increase the biomass and carbon-to-nitrogen ratio of plant leaves. Here we demonstrate, for the first time, that leaf changes induced even at moderately increased carbon dioxide greatly enhance the sporulation of A. alternata, a widely distributed allergy-inducing fungus. This enhancement of spore production may be a contributing factor in the increasing prevalence of asthma.